John H. Helsdon

THE SEVERE THUNDERSTORM ELECTRIFICATION AND PRECIPITATION STUDY

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

An examination of thunderstorm‐charging mechanisms using a two‐dimensional storm electrification model

The early, prelightning, electrification of a storm resulting from noninductive (NI) charging involving graupel, cloud ice/snow, and supercooled cloud water in a riming environment is studied using a comparative approach in a two-dimensional storm electrification model. The primary schemes examined are NI charge transfers based on the laboratory work of Takahashi [1978] and Saunders et al. [1991]. The NI mechanism, based on Takahashis work, develops a positive dipole (positive charge above negative) and electric fields approaching 185 kV m−1 as the cloud enters the dissipating stage. Charge transfers, based on the work of Saunders and colleagues, had to be reduced in magnitude to produce electrification that is consistent with the observations. In addition, the Saunders scheme produces an initially inverted dipole (negative charge above positive) which resolves to a positive dipole in the latter part of the simulation and produces electric fields approaching 250 kV m−1. Sensitivity tests show that the NI scheme, based on Takahashis work, is sensitive to the number concentration of ice crystals, whereas the Saunders-based scheme is much less sensitive to ice crystal numbers. The Saunders parameterization has strong positive charging of graupel at low effective liquid water content and low temperature. This positive charging can result in an unusual cloud-top charge structure when used at full value but is benign when the charging is reduced in magnitude. The charge structure resulting from the Saunders scheme is quite sensitive to the calculation of the effective water content, which determines the level of charge reversal. Both of the NI schemes are capable of producing electrification that approaches thunderstorm levels.

An intracloud lightning parameterization scheme for a storm electrification model

The parameterization of an intracloud lightning discharge has been implemented in our Storm Electrification Model. The initiation, propagation direction, and termination of the discharge are computed using the magnitude and direction of the electric field vector as the determining criteria. The charge redistribution due to the lightning is approximated assuming the channel to be an isolated conductor with zero net charge over its entire length. Various simulations involving differing amounts of charge transferred and distribution of charges have been done. Values of charge transfer, dipole moment change, and electrical energy dissipation computed in the model are consistent with observations. The effects of the lightning-produced ions on the hydrometeor charges and electric field components depend strongly on the amount of charge transferred. A comparison between the measured electric field change of an actual intracloud flash and the field change due to the simulated discharge shows favorable agreement. Limitations of the parameterization scheme are discussed.

Numerical modeling of lightning‐produced NOx using an explicit lightning scheme: 1. Two‐dimensional simulation as a “proof of concept”

[1] We use the two-dimensional (2D) version of our Storm Electrification Model to test its potential for studying lightning-produced NO x . We assume that NO production is a function of energy dissipation and calculate this value from the electric field before and after each lightning flash. We use a production rate of 9.2 x 10 16 molecules joule -1 to generate the NO. Using a limited set of chemical reactions involving NO, NO 2 , and O 3 , we simulated a small storm with 10 intracloud lightning flashes produced over a 2-min span. Their energy dissipation ranged between 0.024 and 0.28 GJ. The simulation was run an additional 18 min after the cessation of lightning. Our results show that the parameterization produced NO mixing ratios internal to the cloud of the order of 10 ppbv after the most energetic flashes and 1-2 ppbv in the upwind portion of the anvil toward the end of the simulation. These mixing ratios are shown to be comparable to observations in a generic sense. Comparison with the C-shaped profiles developed by Pickering et al. [1998], also using a 2D model, show similarities, but our results are more weighted toward larger values at higher altitudes than those of Pickering et al. This may be due to differences in the length of the simulation, a lack of cloud-to-ground lightning in our work, a lack of reactive chemistry in Pickering et al., or the use by Pickering et al. of the assumption of Price et al. [1997] that intracloud flashes dissipate one tenth the energy of cloud-to-ground flashes. We show, using recent observational data and an analysis of the assumptions of Price et al., that this one tenth energy dissipation assumption is not appropriate. We conclude that our use of an explicit lightning scheme to study NO production at the process level is a viable methodology.

Precipitation development and electrification in Florida thunderstorm cells during Convection and Precipitation/Electrification Project

Precipitation development and electrification in Florida thunderstorms are observed using an instrumented aircraft and a multiparameter radar. A low concentration of raindrops initially develops in the updraft, and these raindrops begin to freeze when they are carried above the 0°C level. High concentrations of ice particles and downdrafts soon appear in the -5° to -10°C regions of the cloud, where the aircraft penetrated, as do electric fields in the range of tens of kilovolts per meter. In a cell with relatively weak updrafts, drops start to freeze at temperatures just below 0°C. Although significant electric fields are measured by the aircraft, no lightning is observed in this cell. In more vigorous cells, drops first begin to freeze at temperatures between -5°C and -10°C. The electric fields measured by the aircraft in these cells are similar in magnitude to those in the weaker cell, but lightning is observed in these more vigorous cells. The net charge in convective regions at altitudes just above the aircraft penetration levels, 6-7 km, appears to be negative.

Upward lightning flashes characteristics from high‐speed videos

One hundred high-speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high-speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud-to-ground flashes.

Observations of bidirectional lightning leader initiation and development near positive leader channels

Based on the analysis of high-speed optical and electric field change data, we present three observed cases in which a naturally occurring bidirectional lightning leader initiated and developed in virgin air near a previous established positive leader channel. Twice a new leader formed near an upward propagating positive leader that had initiated from a tower during an upward flash and once a new leader formed near a downward propagating positive leader prior to a positive cloud-to-ground return stroke. There were clear and consistent behavioral differences between the positive and negative leader ends of the newly formed bidirectional leader, and the positive end grew more slowly than the negative end in each case. In all three cases, the negative end of the bipolar leader connected with the previously formed positive leader channel creating a new positive leader branch. These rare observations show the bidirectional nature of naturally occurring lightning and suggest that positive leaders can gain branches by connection with newly formed bipolar leaders.

Triggered upward flashes: Analysis of positive cloud-to-ground waveforms

Upward flashes are a subject of study and research since 1939 [1]. The recent increasing number of tall buildings, wind turbines and telecommunication towers has also increased the need of a better understanding of these lightning flashes, particularly, how they are triggered. Upward flashes may be self-triggered or triggered by some previous electrical activity (intracloud or cloud-to-ground flashes). In Brazil, upward flashes are a recent study subject. The first upward flash was observed in January 2012. All cases observed since then were preceded by some lightning flash. The same situation was observed in upward flashes recorded in Rapid City during 2011-2013 which are included in our dataset. To investigate upward flashes we use high-speed cameras (1,000 up to 100,000 frames per second), electric field sensors and lightning location system data. In this paper, waveforms of fast electric field sensors of positive cloud-to-ground flashes which triggered upward lightning flashes were analyzed and compared with high speed cameras observations.

Further Comment on “Examining the Scientific Consensus on Climate Change”

The feature article “Examining the scientific consensus on climate change,” by Peter Doran and Maggie Kendall Zimmerman (see Eos, 90(3), 20 January 2009), while interesting, has a primary flaw that calls their interpretation into question. In their opening sentence, the authors state that on the basis of polling data, “47% [of Americans] think climate scientists agree… that human activities are a major cause of that [global] warming….” They then described the two-question survey they had posed to a large group of Earth scientists and scientifically literate (I presume) people in related fields. While the polled group is important, in any poll the questions are critical. My point revolves around their question 2, to wit, “Do you think human activity is a significant contributing factor in changing mean global temperatures?” Note that the opening sentence of their article uses the phrase “major cause” in reporting the results of the polling, while the poll itself used the phrase “significant contributing factor.” There is a large difference between these two phrases.

Pseudo Color Densitometer Analysis-the Apollo 17/Saturn V Exhaust Plume.

Spectra of the Apollo 17/Saturn V exhaust plume have been obtained in the uv (300ndash;400 nm), visible (400-650 nm), and ir (750-790 nm) regions. Analysis of these data with a pseudo color densitometer reveals (1) a standing wave pattern in the exhaust plume characterized by a wavelength of 9 m, (2) a region of intense continuum within 40 m of the exit plane which supports previous reports of a continuum blackbody source with a peak temperature near 2600 K, (3) a region of continuum emission beyond 40 m that is not blackbody, and (4) line emissions beyond 40 m attributed to the sodium D lines and potassium. It is suggested that an interference filter centered on the sodium D lines could be used on a high speed framing camera to study the turbulent structure of the plume in the nonblackbody region.